1. Field of the Invention
The present invention relates to a laser processing machine for use in, for example, sheet material processing or the like and, more particularly, to the technique of enabling a diameter of a beam incident on a condensing lens in the laser processing machine to be changed.
2. Description of Related Art
The laser processing machine of a fiber light guide type is basically of a structure shown in FIG. 7 of the accompanying drawings. Referring to FIG. 7, a laser beam LB outputted from a laser oscillator emerges outwardly from a point of emergence P through a transmission optical fiber 5, is then condensed by a condensing lens 8 after having passed through a collimating lens 7 and is finally radiated onto a work W to be processed. Since the laser beam LB has an energy density that is high at a portion thereof where the beam diameter is small, the work W to be processed is processed at a location in the vicinity of a focal position (processing position) Q at which the beam diameter is minimal. Given the same angle of divergence of the laser beam LB from the point of emergence P, the beam diameters at the focal position Q, that is, spot sizes S1 and S2 (shown respectively in FIGS. 8A and 8B), and associated Rayleigh lengths R1 and R2 (similarly shown respectively in FIGS. 8A and 8B) are determined in dependence on the corresponding diameters D of the laser beam LB incident on the condensing lens 8 (the diameter of the laser beam LB incident on the condensing lens 8 being hereinafter referred to as “incident beam diameter”). Each of the Rayleigh lengths R1 or R2 is the distance as measured in the optical direction, which is about equal to √2 times of the radius of the associated minimum spot diameter S1 or S2 and the work W is desirably processed within this range.
FIGS. 8A and 8B illustrate respective diagrams showing the relation between the incident beam diameter and the spot diameter and the Rayleigh length. If as shown in FIG. 8A the incident beam diameter D1 is large, the spot diameter S1 is small and the Rayleigh length R1 is short. Conversely, if as shown in FIG. 8B, the incident beam diameter D2 is small, the spot diameter S2 is large and the Rayleigh length R2 is large. Accordingly, with respect to the sheet metal having a relatively small thickness, a processing with a high efficiency is performed with the large incident beam diameter and at the high energy density. With respect to the sheet metal having a relatively great thickness, if the incident beam diameter is reduced to a relatively small value, the processing can be enabled within the large Rayleigh length R2 even with the sheet metal of the great thickness although the energy density is low.
Following methods have hitherto been available to change the incident beam diameter.
One of those methods is to use, in a laser processing machine of a type utilizing a carbon dioxide gas laser oscillator as a laser beam source, a variable curvature mirror provided, separate from a plurality of transmission mirrors, on the laser beam transmission optical path leading from the oscillator to the condensing lens. By changing the curvature of the variable curvature mirror, the diameter of the laser beam incident on the condensing lens can be changed. In this respect, see, for example, the patent document 1 listed below.
Another one of the methods is to use a collimating lens intervened between the point of emergence and the condensing lens so that, by shifting the collimating lens in a direction parallel to the optical axis, the diameter of the laser beam incident on the condensing lens can be changed. In this respect, see, for example, the patent document 2 listed below. It is to be noted that the collimating lens is primarily used to convert laser rays of light, which have been diverged from the point of emergence, into parallel rays of light.